In aerospace structures such as around the fastener holes, etc, multiple small cracks can emerge concurrently. These small cracks can elude inspections because of their small sizes. Sudden coalescence of these small cracks, however, could lead to a large crack in a short period of time, which may lead to catastrophic structural failures. The goal of this project is to develop a sensing mechanism that can track multiple crack growth with millimeter resolution. An innovative sensing concept of using a microstrip patch antenna for crack growth detection was investigated. The major achievements of the project are: 1) validated the effect of crack growth and crack orientation on the resonant frequencies of the microstrip antennas; 2) demonstrated wireless interrogation of the unpowered microstrip antenna sensors; 3) realized multi-sites crack detection using a four-antenna sensor array. Techniques to fabricate the microstrip antenna on flexible polyimide substrate as well as the associated interrogation circuit were developed. Numerical simulation of the antenna sensor was developed and validated with experiment results. Control software and data processing algorithm were developed to automate the wireless interrogation process.